Make UINode's key a String

sky/sdk/lib/editing2/input.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky' as sky;
 
 import '../widgets/basic.dart';
 import 'editable_string.dart';
 import 'editable_text.dart';
 import 'keyboard.dart';
 
 typedef void ValueChanged(value);
 
 class Input extends Component {
 
-  Input({Object key,
+  Input({String key,
          this.placeholder,
          this.onChanged,
          this.focused})
       : super(key: key, stateful: true) {
     _editableValue = new EditableString(
       text: _value,
       onUpdated: _handleTextUpdated
     );
   }
 
@@ skipping 67 matching lines @@
     );
   }
 
   void didUnmount() {
     if (_isAttachedToKeyboard)
       keyboard.hide();
     super.didUnmount();
   }
 
 }